This invention generally relates to engine couplings, and is specifically concerned with an improved, low-inertia coupling assembly for reducing stress in the interface between the ends of a crankshaft and a drive train.
Coupling assemblies for transmitting rotary power from the working end of a crankshaft of an internal combustion engine to a driven shaft are well known in the prior art. Such coupling assemblies generally comprise a high inertia flywheel in combination with a flexible coupling which interconnects the working end of the crankshaft with the driven shaft. The flexible coupling may include a resilient member formed from an elastomeric material. Such couplings are most typically used in diesel engines, and the primary purpose of the high-inertia flywheel is to smooth out the amplitude of the torque generated by the working end of the crankshaft. A secondary purpose of the flywheel is to provide a mount for the ring gear which engages the output gear of the starter motor of the engine. The flexible coupling utilized in such prior art assemblies not only serves the function of mechanically interconnecting the working end of the crankshaft with a driven shaft; the flexibility provided by the elastomeric material in the coupling advantageously dampens impulse torques which might otherwise be generated between the crankshaft and the driven shaft. Such unwanted impulse torques may occur, for example when the driven shaft is a cardan-type shaft, and the elastomeric material provided in such a flexible coupling allows the coupling to drive such a shaft for a maximum amount of time without failure.
While such prior art coupling assemblies have performed satisfactorily in the past, the applicant has observed a number of shortcomings in the performance of such couplings as the power of diesel engines has increased over the years. For example, the applicant has noted that the relatively large inertias associated with the flywheels of prior art couplings (typically between 150 and 400 lbs*ft.sup.2 in diesel engines of between about 500 and 2000 horsepower) tend to cause the node of the first mode of crankshaft torsional vibration to be located in the vicinity of the flywheel itself. Such a location has the effect of maximizing the amplitude of the torsional vibration experienced by the free end of the crankshaft. Since the free end of the crankshaft of such diesel engines is typically connected to an accessory drive train such as the timing gear train and vehicle accessory drives, the relatively large amplitude of torsional movement of the free end of the crankshaft creates undesirable stress in this gear train which is particularly intense with respect to the teeth of a crank nose pinion of the gear train.
The applicant has also observed three other major problems that come about as a result of the relatively large mass of the flywheels used in such prior art couplings. The first and most important of these problems is concentration of intense stress on only a few of the gear teeth of the gear train driven by the free end of the crankshaft. The inherent natural frequency of the crankshaft mode (or second system mode) of torsional vibration causes the crankshaft to be excited by relatively low engine orders (such as the second, third or fourth orders in a four, six or eight throw diesel crank respectively). Hence, in the case of an eight throw crank, the excitation of the crankshaft mode of torsional vibration in the engine of a prior art flywheel assembly by the fourth engine order results in the same four teeth (located 90.degree. apart) being subjected to very high torsional vibrational stresses with each revolution of the gear wheel. After a period of time, these stresses cause these four gear teeth to fail, thereby necessitating an expensive and time-consuming replacement of the gear wheel. A second problem associated with the use of a high inertia flywheel in such prior art couplings is the relatively low frequency it confers on the coupling mode (first system mode); i.e. frequencies in the range of 15 to 20 hertz. While these frequencies avoid major exciting orders in the engine operating speed range, the engine has to run through the coupling resonance speed during start-up, and can damage the coupling by excessive deflections at such low frequencies.
A third problem occurs if this low coupling mode frequency brings the half-order resonance speed within the upper speed range of the engine. Either a misfiring cylinder, or vigorous governor action will cause a high level of half-order excitation which can damage or break the coupling under these conditions.
Other shortcomings associated with the use of such a high-inertia flywheel include the out-of-balance and bending moment forces that such a flywheel applies to the crankshaft which supports it, as well as the expense necessitated by the precision manufacture and installation of such heavy components in an engine.
Clearly, there is a need for an improved coupling assembly which overcomes the shortcomings and problems associated with the use of high-inertia flywheels in such assemblies.